1. Technical Field of the Invention
The present invention relates generally to an automotive automatic transmission controller equipped with a feature to increase an engine power for minimizing a shift shock occurring during downshift control executed according to a driver's deceleration intention to develop the engine braking.
2. Background Art
Most modern automotive automatic transmissions are designed to switch between hydraulic control lines to establish engagement and disengagement of frictional elements such as clutches and brakes selectively for achieving a desired one of speeds. When the accelerator of the vehicle is released fully during running on a downhill, but the engine braking is insufficient, the driver of the vehicle usually turns off the overdrive switch or moves the shift lever from the D to S or L range to make a downshift for increasing the engine braking.
Such a downshift to increase the engine braking with the accelerator released will result in an increased gear ratio of the automatic transmission, thus elevating the speed of the engine. In such a running mode of the vehicle to develop the engine braking, the throttle valve is usually closed, so that the torque is transmitted from an output shaft of the transmission to the engine through some of the frictional elements working to establish an after-downshift speed to increase the speed thereof. This may cause the time required for the downshift to be increased, thus giving rise to a delay in the engine braking or inertia torque arising from the increased speed of the engine to appear as braking torque, which results in an instantaneous increase in the engine braking, leading to a large-scale shift shock. A rapid rise in torque transmitted through the frictional elements of the automatic transmission under hydraulic control will result in a quick rise in speed of the engine, so that the downshifting time decreases, but however, the braking torque increases excessively, thus resulting in a more increase in the shift shock.
Japanese Patent No. 2924463 teaches a transmission control system for avoiding the above problems. This system includes a function to increase the power of the engine temporarily upon a downshift to the gear of the automatic transmission which creates the engine braking with the accelerator released and a timer which measures the elapsed time from an output of a gear shifting command signal to switch between hydraulic lines to establish a downshift of the transmission. The system works to activate the engine power increasing function based on the elapsed time, as measured by the timer, in order to rise the speed of the engine during an interval between start of slipping of a high-speed one (i.e., an off-going clutch) of the frictional elements of the automatic transmission which is to be disengaged to achieve the downshift and completion of engagement of a low-speed one of the frictional elements which is to be engaged to achieve the downshift. The system determines the time the engine power increasing function should be initiated as a function of an operating parameter of the vehicle, such as the oil temperature in a hydraulic control circuit of the automatic transmission or the speed of the engine, which is a factor of at least one of lags in engagement or disengagement of the frictional elements and increase in the power of the engine.
Usually, there are time lags in completion of engagement or disengagement of the frictional elements of the automatic transmission and between start of control to open the throttle valve and an actual rise in power of the engine. The system also works to determine the time the engine power increasing control should be initiated so as to compensate for the time lags for reducing the shift shock and shortening the shifting time. The above patent also teaches control of opening the throttle valve so that the engine blows up at a time a high-speed one of the frictional elements starts to slip.
Japanese Patent First Publication No. 7-247874 also teaches techniques to launch the engine power increasing control before start of an actual gear shift of the automatic transmission for compensating for a time lag inherent in the engine power increasing control.
The above systems are, as already described, designed to launch the engine power increasing control during an interval between start of control of hydraulic pressure on the automatic transmission for achieving a downshift and start of an actual downshift in the automatic transmission in order to compensate for the time lag between start of control to increase the power of the engine and an actual rise in power of the engine. The actual downshift is accomplished by lowering the torque transfer capacity of an off-going one of the frictional elements (i.e., the hydraulic pressure acting on the off-going one) which works to transfer the torque before the downshift. Therefore, if a defect occurs in a hydraulic controller working to control the torque transfer capacity, it will result in an error in dropping the hydraulic pressure acting on the off-going frictional element and a difficulty in achieving the downshift. For instance, in a case of a 4-3 downshift, the power of the engine is increased while a 4th speed gear ratio is locked without developing the engine braking, thus resulting in undesirable acceleration of the vehicle during the hydraulic control for the downshift.